There is a great need to determine the condition of certain electrical equipment such as generators and motors. High voltage generators such as are used by electric power utilities are subject to faults and deterioration which can be relatively easily corrected if detected sufficiently early but which can lead to catastrophic failure if left uncorrected. A variety of techniques have been employed to monitor the condition of such equipment in order to evaluate whether or when maintenance should be performed. Such techniques generally measure, directly or indirectly, the quality of the electrical insulation of the equipment. A high voltage generator, such as a 13.2 kV generator used by an electrical utility, includes insulated conductors which are disposed in slots in laminated steel structures. During generator operation, the insulation is subjected to a very large cyclic compressive load. This is a result of the mechanical forces generated by the interaction between the electrical current in the conductor and the magnetic field. Such stresses, together with time and environmental conditions, eventually result in deterioration of the insulation sufficient to permit corona generation. Corona is a localized electrical discharge in a gaseous region adjacent an electrical circuit which occurs due to transient ionization of the gas when the voltage stress in the gas exceeds a critical value, for instance an electric field of about 3000 V/m in air. While corona can arise due to insulation degradation, the corona itself causes further insulation degradation and accelerates the process toward complete equipment failure. While certain measures can be taken to suppress corona, such as applying semiconductive paint over the insulation, they generally merely extend the time until corona occurs.
Corona is a serious problem, and can lead to catastrophic failure of a generator. Accordingly, generator condition monitoring has included corona detection and monitoring of corona-related conditions. For the most part, prior corona detection techniques have required the generator to be taken out of service, which is a substantial disadvantage. One technique is to visually inspect the generator windings to detect the effects of corona on the insulation; corona may leave a white residue. Visual inspection may require substantial disassembly of the generator to access locations where corona may occur, and the visual evidence of corona may be overlooked. Another corona detection technique which requires the generator to be out of service is electrical measurement of the insulation electrical characteristics from which inferences regarding the insulation quality and susceptibility to corona may be drawn. D.C. potentials may be applied to the windings for measurements of charging and static resistive currents; A.C. potentials may be applied to the windings to make insulation power factor measurements; or overvoltages may be applied to determine if the insulation can survive them. A further drawback of these electrical testing methods is that they do not provide information regarding the physical location at which an insulation problem may exist; thus they do not provide information to direct repair and maintenance activities to the appropriate locations. Other techniques are used to detect corona based on the effects it produces while it is occurring; these include detecting radio or ultrasonic noise emitted by the corona. These monitoring techniques also require the electrical load to be removed from the generator, and it is still difficult to locate a corona-generating region using them. Coronas generate ozone, and the telltale odor of ozone has long been used as an indicator of corona. It is difficult to quantify the degree of insulation deterioration and to localize a deteriorated area by detecting corona-generated ozone.
Corona also generates light, and corona detectors which acquire and detect light have been used in research studies regarding corona physics. However, to date optical corona detectors have not been available which can detect corona in an operating electrical machine such as a generator and determine the location where the corona exists.
Utility generators can be taken off line for inspection, testing, maintenance, and repair in the spring and fall, when demand is relatively low, but it is inconvenient and expensive to do so in the summer and winter peak demand seasons. Accordingly, the generators are typically kept on line during the peak seasons, and any minor insulation defects which are overlooked or are incipient and undetectable in the spring and fall can grow to serious or catastrophic problems during the following peak season. Such problems are exacerbated by the fact that there is presently negligible construction of new power plants in the United States; old plants and their generators are therefore being kept on line indefinitely, and the aging equipment is increasingly susceptible to insulation deterioration. When a catastrophic generator failure occurs, it can require eighteen months off line and tens of millions of dollars to repair.
Accordingly, there is a great need for a system for continuously monitoring the condition of a generator while it is in normal operation, which can detect and evaluate the severity and location of corona and related conditions, so that problems can be identified early, monitored during their evolution, emergency action can be taken if necessary, and otherwise repair and maintenance can be scheduled when appropriate, performed efficiently and effectively, and orderly utility operations maintained.